Type IV collagen is an activating ligand for the adhesion G protein–coupled receptor GPR126 by Kevin J. Paavola, Harwin Sidik, J. Bradley Zuchero, Michael Eckart, and William S. Talbot Sci. Signal. Volume 7(338):ra76-ra76 August 12, 2014 ©2014 by American Association for the Advancement of Science

Type IV collagen binds specifically to the N-terminal region of Gpr126. Type IV collagen binds specifically to the N-terminal region of Gpr126.(A) Schematic diagram showing the conserved domains of Gpr126 including signal peptide (SP), CUB (complement, Uegf, Bmp1), pentraxin (PTX), hormone-binding domain (HBD), GAIN domain, and 7TM region in the wild-type protein and the corresponding N-terminal fusion protein Gpr126-NT-Fc. The amino acids mediating the autocatalytic cleavage of the GAIN domain are noted (HLT), and the position of the cleavage is indicated by the line in the GAIN domain. (B and C) Conditioned medium containing Gpr126-NT-Fc fusion protein was mixed with biotinylated collagen or BSA and incubated with streptavidin-agarose. Interactions (B) were detected by immunoblot using an antibody against Fc [Anti-Fc-HRP (horseradish peroxidase)], and total input (C) was detected as a control. (D and E) In the reciprocal pull-down experiments, biotinylated collagen or BSA protein was mixed with Gpr126-NT-Fc fusion protein in conditioned medium and incubated with protein A–agarose. As in (B) and (C), interactions and total input were detected by immunoblot using an antibody against biotin. Blots are representative of three experiments. Kevin J. Paavola et al., Sci. Signal. 2014;7:ra76 ©2014 by American Association for the Advancement of Science

Type IV collagen stimulates cAMP signaling in cells expressing Gpr126. Type IV collagen stimulates cAMP signaling in cells expressing Gpr126.(A) cAMP concentration in rat Schwann cells treated with PBS or of type IV collagen (3 μg/ml) for 1 hour. Data are mean fold change ± SEM from three experiments. **P = 0.0013. (B) cAMP concentration in HEK293 cells transfected with empty vector or Gpr126 and treated with PBS or type IV collagen (3 μg/ml) for 1 hour. Data are mean fold change ± SEM from five experiments. ***P < 0.0004. (C) cAMP concentration in Gpr126-transfected HEK293 cells treated with type IV collagen (3 μg/ml) for up to 480 min. Data are mean fold change ± SEM from two experiments. (D) Fold cAMP concentration in Gpr126-transfected HEK293 cells treated for 1 hour with increasing doses of type IV collagen. Data are mean fold change ± SEM from two experiments. EC50 value, 0.2 μg/ml. (E) cAMP concentration in HEK293 cells transfected with Gpr126 treated for 1 hour with PBS or collagen I, II, III, IV, or V (3 μg/ml) for 1 hour. Data are mean fold change ± SEM from three experiments. **P = 0.0035. Kevin J. Paavola et al., Sci. Signal. 2014;7:ra76 ©2014 by American Association for the Advancement of Science

A region of Gpr126 containing CUB and PTX domains mediates the high-affinity interaction with type IV collagen. A region of Gpr126 containing CUB and PTX domains mediates the high-affinity interaction with type IV collagen.(A) Schematic diagram depicting truncated versions of the Gpr126-NT-Fc fusion protein either containing (CUBPTX-Fc) or lacking (ΔCUBΔPTX-Fc) the CUB and PTX domains. (B) Pull-down experiments using conditioned medium containing Gpr126-NT-Fc, ΔCUBΔPTX-Fc, or CUBPTX-Fc incubated with biotinylated type IV collagen and streptavidin-agarose. Blot is representative of four experiments. (C and D) Biacore sensograms detecting the interaction of type IV collagen with Gpr126-NT-Fc and CUBPTX-Fc. SPR monitored the binding of various concentrations of fusion proteins to immobilized collagen IV on a biosensor chip. The association and dissociation constants (Ka and Kd, respectively) for each trial are shown in the figure panels. Data are representative of two experiments; the values for association and dissociation constants are reported as means ± SD in the text. Kevin J. Paavola et al., Sci. Signal. 2014;7:ra76 ©2014 by American Association for the Advancement of Science

The N-terminal region of Gpr126 inhibits receptor signaling activity. The N-terminal region of Gpr126 inhibits receptor signaling activity.(A) Schematic diagram showing the C-terminally FLAG-tagged constructs representing full-length (Gpr126-FLAG) and N-terminally truncated Gpr126 (Gpr126ΔNT-FLAG). (B) Upper panel shows FLAG immunoblot analysis of surface expression and total lysate of HEK293 cells transfected with tagged full-length [wild-type (WT)] or N-terminally truncated (ΔNT) Gpr126. Lower panel shows control blot probed for actin, a cytoplasmic protein. Blot is representative of two experiments. (C) cAMP concentration in HEK293 cells transfected with empty vector, Gpr126-FLAG, or Gpr126ΔNT-FLAG and treated with PBS or type IV collagen (3 μg/ml) for 1 hour. Data are mean fold change ± SEM from five experiments. **P = 0.0037; ***P = 0.0001; N.S., P = 0.36. Kevin J. Paavola et al., Sci. Signal. 2014;7:ra76 ©2014 by American Association for the Advancement of Science

The gpr126st86 mutation truncates Gpr126 after the PTX domain and disrupts myelination and ear development. The gpr126st86 mutation truncates Gpr126 after the PTX domain and disrupts myelination and ear development.(A) Schematic representation of Gpr126 showing conserved domains and the mutations in gpr126st86 and previously characterized gpr126st49 (26). (B) Sequence chromatogram of heterozygous sibling and gpr126st86 mutant. STOP in the amino acid code marks a premature stop codon in the mutant. (C) Expression of mbp in WT (gpr126st86/+) and mutant (gpr126st86/st86 and gpr126st86/st49) larvae. Arrowheads mark mbp (or lack of it) in the lateral line nerve at 5 days after fertilization. (D) Ear morphology in maternal (M) gpr126st86/+ mutants (which have WT morphology), maternal-zygotic (MZ) gpr126st86/st86 mutants, and zygotic (Z) gpr126st86/st86 mutants. Arrowheads mark the normal pericardium; arrows mark ears. Scale bars, 100 μm. Kevin J. Paavola et al., Sci. Signal. 2014;7:ra76 ©2014 by American Association for the Advancement of Science